Recently, study and development has been made regarding decrease in thickness and weight of a note board type personal computer, enlargement of a display or high precision of image. In parallel with this development, decrease in thickness, increase in width and improvement of quality of a polarizing plate protective film has been strongly required. Generally, a cellulose ester film is widely used as the polarizing plate protective film. The cellulose ester film is wound around a spool to form a film roll, which is stored or transported.
As the display panel size is enlarged, a film roll with large width and a long length has been required. However, such a film roll has problem in storage stability. There is, for example, problem that the films are adhered to each other to deform or the films form a convex shape as if there is foreign matter between the films. In order to overcome such a problem, a method has been proposed in which microparticles are added to a film (see Japanese Patent O.P.I. Publication No. 2001-114907). However, this method has problem in that when the amount of microparticles added to cellulose ester film is increased, it increases haze of the cellulose ester film, resulting in lowering of transparency.
Recently, as demand to high image quality is strong, a film without foreign matter and with high transparency has been required. It is difficult to obtain a cellulose ester film with improved sliding property and high transparency by increasing an amount of microparticles to be added to a cellulose ester film.
The present inventor has found that in the above method, the film, in which 10-20 nm microparticles are used as secondary particles, have many protrusions on the film surface, but protrusions contributing to sliding property are a few, and protrusions, which do not contribute to sliding property, increase haze.
In order to overcome the problem, a method is proposed in which a center-line average surface roughness Ra of film is limited (see Japanese Patent O.P.I. Publication No. 2003-326542). Ra is a parameter which is not influenced by a distance between protrusions of the film surface or a density of the protrusions each being an important factor when the film contacting each other is stored for a long period. Therefore, there is problem in that the same Ra value may provide different effects in sliding property, haze or storage stability.